Glycerol-3-phosphate cytidylyltransferase (G-CT) is a 15 kD enzyme involved in teichoic acid biosynthesis in gram positive bacteria. In addition, this bacterial enzyme is 33% identical to the catalytic region of the mammalian cytidylyltransferases that are essential in phosphatidylcholine biosynthesis. This structure should play a major role in furthering mechanistic studies of the cytidylyltransferase reaction that is instrumental in phospholipid biosynthesis. The most promising data set was obtained from crystals of G-CT which diffract to 2.8 _ in space group R3. A mercury derivatized (mercury iodide) data set is 98% complete at three wavelengths (inflection, peak, and remote) with an Rsym on intensities of 6%. The anomalous difference Patterson map and the dispersive difference Patterson map show a single 3s peak in the Harker section. Phasing is being carried out using MADRBE (A. Friedman) and the MADSYS package (W. Hendrickson/W. Weis). Data at different wavelengths were scaled using local scaling in NEWLSC (A. Friedman). The figure of merit using the data from 15 _ to 4 _ is currently 0.59. Efforts are in progress to interpret clearer portions of the electron density and to improve phasing using density modification and phase recombination.